# The Order Book
At the core of Mangrove's trading model are offer lists. Each offer list represents one side of the market, either asks or bids.
Hence, a full market will always feature **two offer lists**. For instance, in a WETH/DAI market:
* WETH-DAI offer list, the asks: offers giving WETH and wanting DAI
* DAI-WETH offer list, the bids: offers giving DAI and wanting WETH
Together, these two lists form the on-chain order book for the market.
> [Mangrove's SDK](https://docs.mangrove.exchange/dev/interacting-with-js/getting-started) offers Market abstractions that allows liquidity providers and takers to interact with Mangrove using standard trading concepts such as *order book*, *base*, and *quote*.
#### Offer List Identification
Each offer list is uniquely identified by a struct called `OLKey`:
```solidity
struct OLKey {
address outbound_tkn;
address inbound_tkn;
uint tickSpacing;
}
```
* **outbound\_tkn** – token given by the offer
* **inbound\_tkn** – token wanted by the offer
* **tickSpacing** – defines which price ticks are valid for that market
For instance, in the WETH–DAI offer list:\
`outbound_tkn = WETH`, `inbound_tkn = DAI`, and `tickSpacing = 1`.
#### How Offers Are Stored
Offers are grouped by their **tick,** a discrete price level, and organized in a FIFO**,** doubly-linked list.\
Within a given tick, offers are ordered by insertion time, so the earliest posted offer at that tick will execute first.
> **Note** :\
> If you haven't done it yet, we strongly suggest that you first get familiar with the [Ticks, ratios, and prices](https://docs.mangrove.exchange/dev/protocol/technical-references/ticks-ratios-and-prices) page.
## Example: WETH/DAI market[](https://old.docs.mangrove.exchange/developers/protocol/technical-references/offer-list/#example-wethdai-market)
Let’s look at both sides of a WETH/DAI market with `tickSpacing = 1`.
#### WETH-DAI (Asks)[](https://old.docs.mangrove.exchange/developers/protocol/technical-references/offer-list/#offer-list-1-weth-dai-asks)
| Tick | Ratio (DAI/WETH) | Offer ID | Gives (WETH) | Gas required | Maker Contract | Offer Gas Price |
| ----- | ---------------- | -------- | ------------ | ------------ | -------------- | --------------- |
| 75171 | 1838.53 | 42 | 0.7 | 220,000 | `0x2468xyz...` | 160 |
| 75171 | 1838.53 | 96 | 1.3 | 280,000 | `0x1357klm...` | 140 |
| 75200 | 1843.87 | 7 | 0.6 | 210,000 | `0x3287opq...` | 190 |
#### **DAI-WETH (Bids)**
| Tick | Ratio (WETH/DAI) | Offer ID | Gives (DAI) | Gas required | Maker Contract | Offer Gas Price |
| ------ | ---------------- | -------- | ----------- | ------------ | -------------- | --------------- |
| -75103 | 0.0005476 | 77 | 925.26 | 250,000 | `0x5678def...` | 150 |
| -75103 | 0.0005476 | 177 | 916.47 | 270,000 | `0x9101ghi...` | 170 |
| -75041 | 0.0005510 | 42 | 871.76 | 300,000 | `0x1234abc...` | 200 |
> For bids, the ratio is the inverse, WETH/DAI instead of DAI/WETH.
Here:
* `tick` represents the discrete price level
* `ratio` is the derived price at that tick (in quote/base units)
* `gives` is the amount of outbound token offered
* `gasreq` and `gasprice` determine execution cost and bounty behavior
#### Key Fields
| **Field** | **Meaning** |
| --------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Offer ID** | Unique identifier of the offer within its offer list. Two offers may share the same ID if they belong to different lists. (see offerId 42) |
| **Gas Required (`gasreq`)** | The maximum gas the offer’s maker contract can consume when called by Mangrove. |
| **Maker Contract** | The address that manages the offer. It can be an EOA or a [maker contract](https://docs.mangrove.exchange/dev/protocol/technical-references/makers), implementing [makerExecute](https://docs.mangrove.exchange/dev/protocol/makers/maker-contract#trade-execution) and [makerPosthook](https://docs.mangrove.exchange/dev/protocol/makers/maker-contract#trade-posthook). |
| **Gas Price (`gasprice`)** | The gas price covered by the offer’s [provision](https://docs.mangrove.exchange/dev/protocol/technical-references/makers/offer-provisions). Must be at least Mangrove’s current gas price when posted. |
We display human-readable amounts in the examples for readability, but on-chain Mangrove only works with raw token values and never uses the `decimals` of a token.
For simplicity, the tokens used in these examples have the same number of decimals (18). When that's not the case, care must be taken to handle decimals, especially for ratios.
See the [Ticks, ratios, and prices](https://docs.mangrove.exchange/dev/protocol/ticks-ratios-and-prices#prices-decimals-and-raw-values) page for a detailed discussion on decimals.
#### Offer List Configuration and Gas Cost
Each offer list has [configuration](https://docs.mangrove.exchange/dev/protocol/technical-references/governance-parameters/data-structures-and-views) parameters controlling how new offers are inserted and how gas usage is calculated. Some parameters are [global](https://docs.mangrove.exchange/dev/protocol/technical-references/governance-parameters/global-variables), while others are [offer list specific](https://docs.mangrove.exchange/dev/protocol/technical-references/governance-parameters/local-variables) (see [Governance](https://docs.mangrove.exchange/dev/protocol/technical-references/governance-parameters) for details).
Because of Mangrove’s fixed tree structure for ticks, the gas cost of posting, updating, or retracting an offer is bounded and predictable. The details are described in the [annotated code](https://old.docs.mangrove.exchange/MgvDoc/) of the Mangrove protocol.
